Systems are known that provide transcutaneous measurement of blood flow rates in vascular accesses of dialysis patients. Such systems are typically relied upon to determine blood flow at access sites to blood vessels. Proper blood flow at the access sites is very important because these sites are limited on a patient. For example, in hemodialysis, typical access sites are located in the arm, leg, or neck of the patient. Needles are placed into the access to facilitate the easy removal of blood on the “arterial” or upstream side of the dialyzer and typically return the purified blood downstream of the first needle placement on the “venous” side. Unfortunately, in many cases, access sites clot or become otherwise compromised over time. This results in decreased blood flow through the access site necessitating either angioplasty or a surgical fix. Sensing a poor access blood flow may help indicate deterioration of the access site, prompting preemptive action on the site to correct the problem with less invasive measures than would likely have been otherwise required.
A sensor used for transcutaneous measurement of blood flow rates typically includes a pad made up of a flexible circuit board loaded with two light emitting diodes (LEDs) and two photodetectors covered with cured silicon. The LEDs and photodetectors are alternately mounted in a straight line with equal spacing between each. This arrangement forms three illuminated “optical zones” in the patient's tissue where light penetrates the skin (i.e., two outer zones separated by a center zone). The depth and diameter of the illumination zones is determined by the spacing of the LED/photodetector pair and the electrical current driving the LED. LEDs radiating light at 800 nm are used because this wavelength of light is absorbed and scattered by hemoglobin (Hb) and is isosbestic with variations in oxygen content.
The outer two “optical zones” of the sensor observe the average Hb content in the normal patient tissue as a result of normal blood diffusion through the skin. The center “optical zone” is positioned over the access site (e.g. a Gortex graft or a natural fistula) where the concentration of Hb is naturally much higher. In essence, the bulk absorption coefficient of the Hb in the access site is referenced to the average of the outer two zone Hb measurements in normal tissue adjacent the access site. Measures from the adjacent normal tissue give a baseline for the blood flow measurement at the access site.
Once the sensor is firmly and correctly positioned, a 30 ml injection of normal saline is shot into a dialysis needle, which is upstream from the sensor pad. As the saline passes under the sensor's center “optical zone,” the Hb drops significantly as the red cells in that zone are temporarily diluted by the saline. This dilution is measured through real time measurements of the bulk extinction coefficient changes during the injection event. Fick's Principle is then applied to the data where the volume of the injection is divided by the integration of the Hb change over the time of the injection to produce the flow rate in the vascular access under test. This method of measuring blood flow rate is explained in detail in U.S. Pat. No. 6,746,407 to Steuer et al., entitled “Method of Measuring Transcutaneous Access Blood Flow.”
These types of systems are useful for detecting compromised blood flow rates at access sites for procedures such as hemodialysis. But they have not previously provided any measurement of the progression of a dialysis procedure or the effectiveness of such a procedure. For example, during hemodialysis it is useful to monitor the amount of fluid removed from the patient's blood. With dialysis accomplished in clinical settings extracorporeal blood can be measured to determine the percentage change in blood volume using a device such as a Crit-Line® Monitor, which is available from Fresenius Medical Care North America, 920 Winter Street, Waltham, Mass. 02451. However, if the patient is treated with peritoneal dialysis, this type of dialysis is most often done outside of clinical settings, and through use of a different process where solutions wash the toxins out of the peritoneum and no extracorporeal blood is available to analyze.